In current LINAC-based IMRT, a critical component that is missing is the real-time monitoring of the tumor position. Furthermore, a treatment verification system capable of documenting the beam targeting and preventing geographic misses is not available, especially for modern modalities such as gated delivery and therapy relying on MLC-based tumor tracking. A strategy of using combined MV treatment beam and on-board kV imaging system for real-time tracking of fiducial markers has been proposed recently. When the fiducials are inside both kV and MV beams, the geometric accuracy of the system is found to be on the order of 1 mm in all three spatial dimensions.
In using the treatment MV beam for on-line imaging, a practical difficulty is that the fiducials may be partially or completely blocked by the MLC at a certain segment (segments) of an IMRT delivery. A potential solution to the problem is to design the IMRT plan in such a way that all or some of the implanted fiducials are inside the segmented IMRT fields provided that the final dose distribution is not compromised excessively.
“Seeing” the fiducial is a general requirement whenever the fiducials need to be monitored during the beam-on time. This issue arises not only in recently available time-resolved radiation therapy, such as respiration-gated IMRT and four-dimensional (4D) IMRT, but possibly also in conventional 3D IMRT where it may be desirable to ensure that the target volume remains within a prespecified tolerance.
What is needed in the art is a method of introducing a soft or hard penalty in an inverse planning calculation to discourage or avoid the blockage of the implanted fiducials in optimized multi-leaf collimator (MLC) segments.